Fine metal powders, especially powders with diameters in the range of approximately 50 to 500 micrometers, are ideally suited for various powder metallurgical applications. Currently, there are many methods employed for producing these fine metal powders. A common powder generation process is gas atomization, in which a high velocity gas stream is employed to disintegrate a molten metal stream. Another technique, referred to as rotary atomization, involves pouring molten metal onto a spinning disk or cup which breaks up the stream and centrifugally ejects the metal as metal droplets; the droplets then solidify into spherical powder particles. Two other related techniques are the rotating electrode process and the plasma rotating electrode process, both of which employ a rotating consumable electrode which is melted with an arc or plasma arc, respectively. Molten metal droplets are flung from the electrode by centrifugal force and solidify as spherical powder particles.
In all of these powder formation techniques, a pure inert gas cooling atmosphere must be provided to produce the pure metal powders generally required for powder metallurgy; because of the high temperature and surface area of the molten metal drops, the drops are extremely prone to oxidation. A typical helium atmosphere must contain less than 10 ppm oxygen to prevent harmful formation of metal oxides.
As an example, in the production of extremely pure nickel-based super alloys such as Rene 95 and MERL 76, the helium comprising the inert cooling atmosphere must have no more than 0.5 ppm oxygen and a dew point of no greater than -100.degree.F. to avoid the formation of oxide shells on the powder particles. If the oxide shells are allowed to form, the surface impurities lead to prior particle boundary decoration in the finished product when the powder is consolidated by hot isostatic pressing (HIP). If even small quantities of the impurities are present, the decorations, which may be carbides nucleated and precipitated at oxide particles, act as sites for fatigue failure. As an example, surface contamination must be avoided in the production by HIP of gas turbine disks designed to run at high rotation speeds, in order to avoid disk fatigue failure. The oxidation problem is also prominent in the production of titanium powders: titanium has a great affinity for oxygen, especially at the elevated temperatures required to produce the molten titanium droplets.
The pure spherical metal powders may be consolidated to form an elongated microstructure by the extrusion process; enhanced component strength may be obtained in the formed parts by the addition of other materials to form metal matrix composites. For example, silicon carbide fibers may be used in fabricating custom metal structures. In making these composites, the silicon carbide fibers may be co-extruded with pure metal powder to form the shapes.